Use of a denture for a long period of time will cause absorption of an alveolar ridge and fit between a mucosal surface of a denture and the mucosal surface in contact with the denture base gradually deteriorates. As a result, application of a local excess pressure to the mucosal surface in contact with the base develops an abnormal condition in the mucous membrane in contact with the denture base or the alveolar ridge. When the mucous membrane in contact with a denture base or the alveolar ridge has something wrong, it is recovered to a normal condition by lining the mucosal surface of the denture with a tissue conditioner or the like, which is a soft resin composition, to relax the pressure added to the mucosal surface in contact with the base from the denture at the time of occlusion.
Such a soft resin composition has a feature that during the use thereof the physical state changes from a paste form to a rubber form via a dough form, i.e., from a viscous fluid to an elastomer, by mixing a powder and a liquid. A mixture of the powder and the liquid is poured onto a mucosal surface of a denture while being in a viscous state and it is installed into an oral cavity while being in a fluid state to fill the gap between the mucosal surface of the denture and the mucosal surface in contact with the denture base. Then, the physical state changes with time to a viscoelastic material having no fluidity or to an elastomer. The soft resin composition which has changed to a viscoelastic material or an elastomer is usually held in the oral cavity in a state of being in contact with the mucosal surface in contact with the denture base usually for about 3 to about 10 days or, if long, for about 30 days.
In soft resin compositions now in the marketed, poly(alkyl(meth)acrylate) is widely used as a powder and mixtures of ethanol with a phthalic ester-based plasticizer typified by dibutyl phthalate (DBP), butyl butylphthalylglycolate (BPBG) and dibenzylbutyl phthalate are widely used as a liquid. Regarding the phthalic ester-based plasticizers, however, a possibility of affecting living bodies as endocrine disruptors has been pointed out and removal thereof has been desired. Ethanol, which is added for increasing a swelling speed of a poly(alkyl(meth)acrylate) when mixing a powder and a liquid, adversely gives stimulation to an oral cavity or leaches into saliva. Ethanol may also cause problems such as deterioration in physical properties of a denture base or deformation thereof because it exhibits swellability also to a denture base in contact.
Patent document 1 discloses a tissue conditioner comprising a mixture of a powder of a copolymer of butyl methacrylate and ethyl methacrylate or a mixture of poly(butyl methacrylate) and poly(ethyl methacrylate) and a liquid of an aromatic carboxylic acid ester such as phthalic ester. According to patent document 1, the tissue conditioner can eliminate displeased feeling of patients during its use caused by stimulation or odor because the tissue conditioner is free from ethanol. However, phthalates and the like are endocrine disruptors, and in other aromatic carboxylate, there are fears of effects on living bodies.
Patent document 2 discloses a soft resin composition for denture bases which comprises (meth)acrylate monomer, an acid ester-based plasticizer, a poly(alkyl(meth)acrylate) and a polymerization initiator. According to patent document 2, in the resin composition, when using an acid ester-based plasticizer instead of phthalic ester-based plasticizer, properties desired for a soft resin composition such as moderate softness and compatibility between a denture base and a mucous membrane in oral cavity can be satisfied without using endocrine disruptors. However, use of an acid ester-based plasticizer unfortunately leads to deterioration in workability due to a low swelling speed of a poly(alkyl(meth)acrylate). In some examples provided in patent document 2, ethanol is added. Further, because an acrylic monomer is used, stimulation by the monomer to an oral cavity often becomes a problem.
Patent document 3 discloses a resin material for a denture base comprising a mixture of a polymerizable monomer having an unsaturated double bond, a poly(alkyl(meth)acrylate) and a polymerization catalyst, wherein at least a part of the poly(alkyl(meth)acrylate) is dissolved in the polymerizable monomer. It is reported that this does not need operations of weighing a powder and a liquid, mixing and kneading them and leaving them at rest for a certain period of time and that the operations can be simplified because it is already in a paste form. It, however, is necessary to harden the material by use of heat, UV rays or the like and therefore it is often unsuitable for taking an impression in an oral cavity.
Patent document 4 discloses a dental plastic pattern material for the production of a cast metal dental clasp or a cast metal denture base, comprising a mixture of a plasticizer such as a vinyl acetate monomer and poly(methyl methacrylate) A plastic pattern having elasticity and restorability can be obtained with sufficient workability by conducting cure polymerization after attaching the pattern material in an unpolymerized or preliminarily polymerized state to a model of the portion to which a cast metal clasp or a cast metal denture base is to be mounted. In a working example, a process for producing a plastic pattern is disclosed, in which a pattern forming material prepared by mixing a vinyl acetate monomer with a powder of poly(methyl methacrylate) to form a paste is attached to a surface of a plaster model and then it is subjected to cure polymerization by heating. The pattern material disclosed in patent document 4 is not one which is installed into an oral cavity, but one which is cured while being attached to a model. For this reason, a highly volatile monomer, such as a vinyl acetate monomer, is preferably used in the pattern material. In a case of installation into an oral cavity, however, use of a volatile monomer is undesirable from the safety viewpoint.    Patent document 1: JP 3-20204 A    Patent document 2: JP 2002-104913 A    Patent document 3: JP 2000-254152 A    Patent document 4: JP 3-112911 A